FIG. 1 is a diagram showing a structure of one embodiment of the conventional automatic focusing apparatus.
In FIG. 1, the reference numeral (1) designates a laser source such as helium-neon laser source; (2) designates a laser ray flux; (3) designates objective lens; (4) designates an object substrate on which a focal point of the flux (2) is formed; (5) designates a beam splitter; (6) designates a condenser lens; (7) designates a pin-hole; (8) designates a ray detector; (9) designates a piezo-electric vibrator for vibrating the pin-hole (7) to the optical axial direction; (10) designates a table on which an optical system is placed; (11) designates an oscillator for exciting the vibrator (9); (12) designates a phase detector which phase-detects an output of the ray detector (8) under rating an output of the oscillator (11); and (13) designates a voice coil type actuator which drives the table (10) by the output of the phase detector (12).
In the apparatus, the laser ray (2) emitted from the laser source (1) is focused through the objective lens on the surface of the object substrate (4). The ray reflected from the surface of the object substrate (4) is focused again through the objective lens (3) thereby separating from the incident path by the beam splitter (5) and the ray is passed to the condenser lens (6) and focused to form ray spot and the ray is passed to the ray detector (8). The pin-hole having a diameter being substantially the same as the diameter of the ray spot is disposed at the focal point of the condenser lens (6) and the pin-hole is vibrated at frequency f by the vibrator (9) excited by the oscillator (11) in the optical axial direction. The output of the ray detector (8) which receives the ray passed through the pin-hole (7) is phase-detected by the phase detector (12) upon rating the output of the oscillator (11) and the detection output is fed to the actuator (13) to drive the table (10) to the arrow line direction A. That is, when the focal point of the objective lens (3) is on the surface of the object substrate (4), the focal point of the condenser lens (6) is at the vibration center of the pin-hole (7) whereby signal having frequency 2f is provided from the ray detector (8) and the output of the phase detector (12) is zero. However, when the surface of the object substrate (4) is deviated from the focal point of the objective lens (3), the position of the ray spot given by the condenser lens (6) is deviated from the vibration center of the pin-hole (7) whereby the component of frequency f is formed in the output of the ray detector (8). The phase corresponds to the direction deviating the surface of the object substrate (4) from the focal point of the objective lens (3). Accordingly, the focal point of the objective lens (3) is automatically focused on the surface of the object substrate (4) by driving the table (10) to the arrow line direction A through the actuator (13) by the output of the phase detector (12) in which the output of the ray detector (8) is phase-detected.
Thus, in the conventional apparatus, it is not always necessary to vibrate the pin-hole (7) itself and the objective lens (3) or the condenser lens (6) can be vibrated. However, it is necessary to form the pin-hole (7). It is not easy to set the pin-hole at the center of the ray spot because the diameter of the pin-hole is preferably about several tens microns. Moreover, the position of the pin-hole may be shifted by outer vibration during long time which causes an erroneous operation of the apparatus. This apparatus is especially not suitable for an apparatus which should be low cost and is used in variable environment such as a video disc regenerative apparatus.